1. Field of the Invention
The present invention relates to a method for cleaning the surface of a semiconductor wafer. More particularly, the present invention relates to a method for completely removing both a photo-resist layer and residual polymers on the surface of the semiconductor wafer after performing an etching process, wherein the photo-resist layer acts as a hard mask during the etching process.
2. Description of the Prior Art
Metal-oxide-semiconductor (MOS) transistors have played a significant role in the integrated circuit industry, and the electrical performance of the gate electrodes of MOS transistors is important to the quality of the MOS transistors. In the process of forming the gate electrodes, a gate oxide layer, a polycide layer and a silicide layer are sequentially formed on the surface of a silicon substrate. Patterns of the gate electrodes are then defined by a photo-resist layer, and the gate electrodes are completed using an etching process. Remaining photo-resist and residual polymers formed during the etching process on the surface of the semiconductor wafer must be removed after the etching process to ensure the electrical performance of the MOS transistors.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 are cross-sectional diagrams of a method for defining a gate electrode 11 on a semiconductor wafer 10 by virtue of a photo-resist layer 20. As shown in FIG. 1, a silicon substrate 12 of the semiconductor wafer 10 is subjected to a thermal oxidation process. The surface of the silicon substrate 12 is oxidized in the thermal oxidation process to form a gate oxide layer 14 with a thickness of about 80 angstroms to 200 angstroms. A low-pressure chemical vapor deposition (LPCVD) process is then performed with silane (SiH.sub.4) as a reaction gas at a temperature of about 575.degree. C. to about 650.degree. C. to form a polysilicon layer 16 with a thickness of about 1000 angstroms to 1500 angstroms over the gate oxide layer 14. A metal silicide layer 18 composed of titanium-polycide is next deposited on the surface of the polysilicon layer 16. The thickness of the metal silicide layer 18 is in a range of about 1000 angstroms to about 1500 angstroms. Thereafter, a photo-resist layer 20 is spin-coated over the metal silicide layer 18 and patterned using a conventional lithographic process to define the gate electrodes.
Next, as shown in FIG. 2, a plasma etching process is performed to vertically etch the metal silicide layer 18, the polysilicon layer 16 and the gate oxide layer 14 that are not covered by the photo-resist layer 20 down to the surface of the silicon substrate 12 so as to complete the gate electrodes 11.
The photo-resist layer 20 has polymeric resins, photo-sensitizers and organic solvents. After the etching process, residual polymers 22, whose composition is still unknown and is under investigation, are usually observed on the walls of the gate electrodes 11 and in the corners between the gate electrodes 11 and the silicon substrate 12. Also, the remaining photo-resist layer 20 must be removed. Consequently, an effective surface cleaning process is necessary to remove both the residual polymers 22 and the remaining photo-resist layer 20 from the surface of the semiconductor wafer 10.
The cleaning process of the prior art method includes: a) performing a dry etching process using an ashing plasma, such as an oxygen plasma; b) performing a wet cleaning process using a fluoride-based solvent, such as EKC 640, or an amine-based solvent. The oxygen plasma in the dry etching process oxidizes and decomposes most of the remaining photo-resist layer 20 and most residual polymers 22 into gaseous CO, CO.sub.2 and H.sub.2 O. The gaseous CO, CO.sub.2 and H.sub.2 O are concurrently extracted using a vacuum system (not shown). After the dry etching process, trace amounts of photo-resist and polymers, most likely organic metal compounds, remain on the surface of the semiconductor wafer 10 after the dry etching process. After performing the wet cleaning process using a fluoride-based solvent, the trace polymers are dissolved. Finally, the solvent and dissolved solutes are removed using deionized (DI) water so as to complete the cleaning of the semiconductor wafer 10.
However, it is disadvantageous to use a fluoride-based solvent or only an amine-based solvent in the wet cleaning process. Although the fluoride-based solvent has significant cleaning abilities, it also slightly corrodes the metal silicide layer 18 and the silicon oxide concurrently formed on the test key on the street in the earlier thermal oxidation process. Manufacturers can not detect the thickness of the remaining silicon oxide directly from the gate electrodes 11. The test key is therefore used as a window from which an optical device can detect the remaining thickness of the silicon oxide in the test key after the cleaning process. The remaining thickness of the silicon oxide is used to monitor and evaluate the efficiency of the entire cleaning process. Once the silicon oxide in the test key has been overly eroded or etched, it is difficult to control the quality of the semiconductor products being manufactured in the subsequent cleaning processes.
In addition, using fluoride-based solvents will cause a serious etching rate problem. Please refer to FIG. 3. FIG. 3 depicts the relationship of thermal oxide etching rate versus bath time. As shown in FIG. 3, the x-axis represents the bath time of the semiconductor monitor wafer (not shown) in a fluoride-based solvent, and the y-axis represents the etching rate of the thermal oxide formed on the surface of the monitor wafer. The etching rate of the thermal oxide using the fluoride-based solvent increases as the bath time increases. This is because the pH value of the fluoride-based solvent decreases as the bath time increases. The increasing etching rate makes the cleaning process unpredictable and exacerbates the erosion of the silicon oxide of the street. On the other hand, an amine-based solvent does not have the above-mentioned problems owing to its pH stability during the cleaning process. However, amine-based solvents show an inadequate cleaning ability for the residual polymers 22 on the surface of the semiconductor wafer 10.